The present invention relates generally to drilling, milling and similar operations performed in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides controlled milling in subsea wells.
It is frequently desirable to isolate a cutting device, such as a drill bit or a mill, from the motion of a tubular string on which the cutting device is carried. For example, where a cutting operation is being performed from a floating rig (sometimes referred to as a “floater”), the tubular string suspended from the floater may rise and fall due to a heaving motion of the rig. Some floaters may be equipped with devices known as heave motion compensators, but these devices are not typically capable of removing all rising and falling motion from a suspended tubular string.
In some circumstances, accurate axial advancement of the cutting device in the well may be required. This accurate advancement is compromised by the rising and falling of the tubular string. For example, the cutting device may be a mill which may be damaged if the mill suddenly impacts a structure downhole. Of course, many other circumstances also require accurate axial advancement of a cutting device, whether the operations are performed from a floater or a land-based rig.
From the foregoing, it can be seen that it would be quite desirable to provide an apparatus which permits accurate axial advancement of a cutting device. It is accordingly an object of the present invention to provide such an apparatus and associated methods of controlling displacement of a cutting device in a well.